Model won't follow its armature in pose mode
try following these steps

after joining all the separate parts and the body mesh together, you should parent the body mesh to the armature by ( in object mode )


selecting the mesh then the armature and pressing Ctrl P
select Armature Deform >> With Automatic Weights
select only the mesh and go to Modifiers
deselect Bone Envelopes in the Armature Modifier





press Tab to enter Edit Mode and navigate to Vertex Groups
ensure only necessary vertices belong to each Vertex Group





press Tab to enter Object Mode
select the armature and press Ctrl Tab to enter Pose Mode
select a bone and move by pressing G or rotate by pressing R


hope this helps!